This invention relates to a process of ion exchange on the surface of glass or glass ceramics.
Ion exchange on glass or glass ceramic based on oxides, in particular based on silicates, by ion exchange of alkali metal ions at the glass surface has found wide industrial application, in particular for chemical hardening.
At temperatures below the transformation temperature Tg, smaller alkali metal ions are replaced by larger ones ("crowding"; "ion stuffing"), which results in compressive stress at the glass surface, thus significantly improving the strength of the glass. The higher the temperature during the ion exchange, the more rapid the ion exchange; however, the temperature selected must not be so high that stress relief can occur in the glass. Conversely, the further the temperature drops below Tg, the longer the time necessary for hardening the surface. A favorable temperature is about 100.degree. C. below Tg (G. H. Frischat, "Ionic Diffusion in Oxide Glasses," Trans Tech Publications, Aedermannsdorf (Switzerland), 1975, p. 75).
In ion exchange above the transformation temperature Tg, it is also possible to produce at the surface glass or even crystals having different (lower) coefficients of expansion, which after cooling lead to a compressive stress at the surface. However, the ion exchange temperature selected must not be so high that the glass can become deformed during the exchange process.
It is furthermore known to produce a coloration in the glass surface by ion exchange, for example, by exchange of alkali metal ions with silver and/or copper ions (see Frischat, p. 83 ff). This method is utilized, for example, when producing scales on glass equipment. The scale is applied to the glass in the form of an AgCl-containing paste; during the ion exchange, silver ions diffuse into the glass and produce therein a substantially permanent coloration. These pastes contain a carrier material which supplies the paste with the necessary cohesion at elevated temperatures (W. Kiefer, Glastechn. Ber., 46 (8), 325 (1973)). Moreover, the carrier material is intended to absorb the exchanged ions. After the coloration step, the paste residues are in some cases very strongly bound to the glass surface which makes their removal difficult, resulting in the risk of damaging the glass surface (microcracks) during the removal.
It is still further known to exchange alkali metal ions in the glass surface for protons, for example, by means of moist sulfur dioxide or trioxide vapors (see, for example, Frischat, p. 88). This exchange improves water resistance and in some cases also the strength.
Many reports on the hardening of glass by means of ion exchange have been published. A summary from a rather theoretical view is the already quoted book by Frischat, in particular the chapter "Chemical diffusion," p. 72-88. One of the industrial processes is represented by German Offenlegungsschrift 1,496,074, in which alkali metal ions are exchanged for lithium ions at a temperature above Tg (600.degree. C.-750.degree. C.). The surface film formed has a lower coefficient of expansion than the remaining glass and, upon cooling, produces the desired compressive stress. According to German Offenlegungsschrift 1,496,470, lithium ions are exchanged in the surface film for larger alkali metal ions at a temperature of about 50.degree. C.-100.degree. C. below Tg, which also results in the desired compression stress. German Auslegeschrift 1,287,763, German Offenlegungsschrift 3,537,561 and U.S. Pat. No. 3,573,072 describe processes for hardening glass ceramic at temperatures below Tg, in which alkali metal ions having a smaller ionic radius are replaced by those having a larger radius. According to German Offenlegungsschrift 1,803,540, Mg.sup.++ and Zn.sup.++ ions in a glass ceramic are replaced in each case by 2 Li.sup.+ ions. The replacement of sodium ions by potassium ions in the surface of a soda-lime glass, followed by removal of alkali from the glass surface is described in Japanese Offenlegungsschrift 55/104,949 (Aug. 11, 1980).
The ion exchange step is generally carried out by means of a molten salt. At low temperatures, molten alkali metal nitrates and nitrites are used. Nitrate and nitrate/nitrite baths can be used up to temperatures of about 450.degree. C. but above this temperature they begin to decompose and attack the surface not only of the glass to be treated but also of the container and other equipment. Working with nitrate and nitrite melts is not without risk and requires special safety precautions due to toxicity and the risk of explosion, which severely limits large-scale industrial use. At higher temperatures, chloride and/or sulfate melts are usually used. Since the melting points of the pure salts are in general too high, eutectics of several salts having the same cations or of several cations having the same anions are usually used. The use of salt melts consisting of a mixture of chlorides and sulfates has the disadvantage that many glasses are attacked at the surface by the melt, since the chlorides are extremely aggressive in this temperature range. The use of salt melts containing two and more cations has the disadvantage that the cations severely obstruct one another. Thus, for example, the exchange of potassium is severely restricted by the presence of sodium ions. What is more, the salt baths undergo aging due to the absorbed ion exchange products absorbed concentrating in the salt bath, which obstructs or weakens the ion exchange. Therefore, the salt baths must be regularly renewed. The disposal of the used salt baths also requires complicated procedures. A further aging-caused disadvantage of the salt baths is that the exchange conditions alter with time. The exchange times and temperatures have to be constantly adapted to these changes, to obtain a product which has consistent properties.